What did Donald Trump do today?

He threatened the mayor of Minneapolis for knowing how federalism works.

Trump posted this to his private microblogging service this morning:

Surprisingly, Mayor Jacob Frey just stated that, "Minneapolis does not, and will not, enforce Federal Immigration Laws." This is after having had a very good conversation with him. Could somebody in his inner sanctum please explain that this statement is a very serious violation of the Law, and that he is PLAYING WITH FIRE!

This is not, in any way, a violation of the law. 

Cities and states are not required to enforce federal law, full stop. This is the essence of federalism, a bedrock principle of American government. It was most recently reaffirmed by the Supreme Court in the 1997 case Printz v. United States, with the majority opinion written by the arch-conservative Antonin Scalia holding that states could not be required even to do incidental clerical work in support of federal law enforcement.

As for whether Mayor Frey is "PLAYING WITH FIRE," federal agents are conducting an orchestrated shock-and-awe terror campaign in Frey's city under the guise of immigration enforcement, specifically because Trump has political grudges against the state's governor and the city's Congressional representative. Among other things, they've killed two Minneapolis residents, shot and wounded a third, kidnapped a five-year-old with valid immigration status and used him as bait, hospitalized an infant child after they threw a tear-gas grenade into a car, dragged an elderly U.S. citizen out into single-digit temperatures in his underwear believing he was a person they already had in custody, and made a point of asking residents to point out "where the Asian people live."

Why does this matter?

• A president who can't or won't understand where his authority ends is either too stupid or too authoritarian to hold office.